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We are currently running our database in RDS - just one master (no read slaves), with a Multi-AZ deployment for fail-over durability.

I am contemplating some performance tweaks, one of which is disabling binary logs. I know they're used for master->slave replication, but I am not sure whether Amazon uses the same mechanism to support its Multi-AZ set-up or not.

I don't want to accidentally break the Multi-AZ feature by disabling binary logs.

You may find in some cases that your Read Replica(s) aren’t able to
receive or apply updates from their source Multi-AZ DB Instance after
a Mulit-AZ failover. This is because some MySQL binlog events were not
flushed to disk at the time of the failover. After the failover, the
Read Replica may ask for binlogs from the source that it doesn’t have.
This loss of MySQL binlogs during a crash is described in the MySQL
document here.

The Multi-AZ feature does not use MySQL's native replication features. Quoting their documentation, "Multi-AZ deployments for Oracle, PostgreSQL, and MySQL DB instances use Amazon technology, while SQL Server DB instances use SQL Server Mirroring." It seems likely that they're doing replication at the storage level and not depending on binary logs. However, disabling binary logs (if RDS will even let you do that) would probably break point-in-time recovery.

Multi-AZ should not be confused with Read Replicas. As @GeekRide's answer shows, Read Replicas do use MySQL's native replication via binlogs.